1. Field of the Invention:
This invention relates to polypropylene sheets for press-through packages employed for packing medicines in the form of tablets, capsules or the like (hereinafter abbreviated as "PT packages" for the sake of brevity), said sheets having excellent transparency, thermoformability and safety, their production process, and polypropylene resin compositions suitable for their production.
2. Description of the Prior Art:
"PT packages" means one form of packages, in which tablets, capsules or the like are separately filled in pockets of a plastic sheet and then sealed with an aluminum foil. They are usually obtained by means of an automatic high-speed packing machine which can continuously perform such steps as thermally forming a thermoplastic sheet, automatically filling tablets, capsules or the like in the thus-formed sheet, sealing them with an aluminum foil, forming perforations or slits and then punching along the contours of intended PT packages.
Hard vinyl chloride resin sheets (hereinafter abbreviated merely as "PVC sheets" for the sake of brevity) have been primarily used for PT packages. The water vapor permeability of PVC sheets are however too large for medicines prone to property changes by moisture such as aspirin, antibiotics and the like. There are thus employed after laminating a resin having still smaller water vapor permeability such as polyvinylidene chloride or the like, leading to a drawback that their product cost is high. It has hence been desired to develop resin sheets having economical prices and smaller water vapor permeability and suited for PT packages.
Although polypropylene sheets have water vapor permeability smaller than PVC sheets, they have problems in transparency and thermoformability and under the circumstances, are not substantially employed in PT packages.
As a method for improving the transparency of polypropylene sheets, it has been known to add a nucleating agent such as dibenzilidine sorbitol or the like to a polypropylene resin. On the other hand, as a method for improving the thermoformability, it has also been known to blend a polyethylene resin with a polypropylene resin. However, these methods have some demerits. For example, the former method results in poor formability while the latter method leads to inferior transparency. It is hence improper to use such resins for PT packages.
As a method for improving both transparency and thermoformability of polypropylene sheets at the same time, it has been reported to blend a hydrogenated petroleum resin with a polypropylene resin (Japanese Patent Laid-Open No. 185635/1983). Polypropylene sheets obtained by this method are suited for PT packages as far as their mechanical and optical properties are concerned. However, they are extracted considerably with normal heptane (hereinafter abbreviated as "n-HT") and many of them do not meet the standard (not more than 150 ppm) of Notification No. 20 of the Ministry of Health and Welfare, Japanese Government. Under the circumstances, they are not usable practically.